Analysis of the AUDI Dakar RS Q e-tron Battery
AUDI has a long experience with Battery development including, of course, for their production vehicles. The AUDI e-tron vehicles have used various battery generations and architectures. When it comes to racing, AUDI record with electrified vehicles includes:
7 seasons of Formula E with a manufacturer title in 2017-2018 season
AUDI R18 etron (Hybrid Vehicle), with 2 overall wins at Le Mans (2013-2014)
Recently, AUDI has changed its marketing strategy abandoning the Formula E but instead entering again Le Mans and more surprisingly racing for the 1st time in January 2022 at the Dakar Rally (in Saudi Arabia).
AUDI wanted to compete with an Electric Vehicle in concordance with their emobility strategy. Nevertheless, it is not yet realistic to race with a fully Electric Vehicle because of the length of daily stages (about 800 km a day) and the lack of infrastructure to fast charge a car in the desert. Note that there is no infrastructure for fuel either, but it is easier to establish a temporary refuelling point.
For this reason, AUDI presented an original concept based on a Range Extender already presented in a previous OROVEL article. In this article, I am presenting further details I collected on the Battery Pack and the platform. Information here below are built on my own research and knowledge. Public information are linked as much as possible and assumptions are supported by relevant explanations.
Dakar Battery Requirements
Left: AUDI RS Q e-tron electric motor (MGU05); Right: real driving conditions. Images credit: AUDI AG
Battery Packs in Motorsport are always different from in-production cars because operating conditions and requirements are very different. This includes, for example, ambient temperature, usage, vibrations, high power discharge (high C-rate, OROVEL Card EV30), etc.
Based on AUDI declarations, the full Dakar project was developed within a few months so that it is very unlikely that they started from a clean sheet but rather based their battery on an existing pack already used in the past.
I believe that it makes sense to assume that the starting point is the AUDI Formula E Battery. Let us remind that AUDI has publicly revealed that the 3 electric motors in the Dakar e-tron are from the AUDI e-tron FE07 Formula E.
But, compared to Formula E, the Dakar Rally puts much more constrain on the vibrations (no smooth roads, jumps in the dunes, etc.) as well as on degradation as the Dakar is very long (about 7,000 km Split into 4,000 km of special stage and about 3,000 km of liaison) over 12 days.
In terms of temperature, the FIA Formula E already tours under very hot climate, notably in Chile or Saudi Arabia. For example, the Santiago E-Prix 2020 registered a track temperatures around 45°C (air temperature 38.3°C).
Battery degradation over the course of the race will be critical as the range extender cannot propel the vehicle: a dead Battery is a standstill vehicle. This will be extremely challenging for the BMS (Battery Management System). The BMS monitors cells that it regroups in clusters depending on the chosen architecture. The BMS ensures that each group of cells is used within optimal conditions (voltage, temperature, etc.) and must compensate for differential cell degradation.
Compared to Formula E, the AUDI Dakar e-tron does not create as much constrain on fast-charging as the Battery is coupled with a Range Extender (the IC Engine) recharging “on the fly”.
Battery Packaging
There are 2 common Battery packaging options in an EV platform: 1) a chest configuration behind the driver / copilot; (2) a skateboard option below the seats. Such a selection influences centre of gravity and packaging.
The AUDI RS Q e-tron includes 2 Electric Motors MGU05, one on each axle. Mass repartition and packaging impose to locate the IC Engine at the back of the car coupled to third electric motor (used as.a generator). Once these elements have been implemented, the battery pack needs to fit into the remaining room in the vehicle. Another additional constrain is that for maintenance and assistance reason, the Battery pack should remain accessible. Also, for security reason, it is preferable that the MSD (Manual Service Disconnection - OROVEL Card EV36) is directly accessible to the driver and/or the copilot.
For all these reasons, the Battery packaging chosen by AUDI Engineers is neither of the common options but a mix configuration: part of the pack is under the seats and the rest is a chest in-between the driver and the co-pilot, where you would normally find the transmission tunnel of a traditional ICE vehicle. The part of the pack under the seats is optimised to provide room for the feet of the driver and the copilot (similar to a Fußgarage used by the Porsche Taycan for the rear passengers). The central mid-chest separator also contains the MSD safety protection easily accessible to the occupants.
AUDI Dakar RS Q e-tron Platform; image credit: AUDI AG
The central location of the battery and the optimised space under the seats allow AUDI Engineers to keep a low centre of gravity and a good mass repartition between the front and the rear of the car. Actually, you can notice that the battery is slightly advanced towards the front of the car simply because the DTM Engine fills the space behind the occupants.
Battery Safety
The Dakar Rally is extremely demanding in terms of shocks and projectiles that can hit the vehicles (small stones but also rocks and of course dust). It is very unusual for Battery Packs to include on purpose a “deformation option”. The Battery Pack needs to be a safe and clean environment and you usual want to avoid any dust and contamination entering the Pack which is more or less sealed.
Of course, specific components (breathing valves / pressure equalisers - OROVEL Card EV38) ensures that a pressure equilibrium between the Battery pack and the ambient air. Nevertheless, acceptable pressure differences are relatively low (around 50 mbar). In case of a rock impact on the battery pack (remember it is seated underneath the vehicle), the Battery Pack volume can be reduced creating too large an overpressure.
Specific venting is another option but if the pressure rise is too high, you put the Battery at risk and the venting would burst open saving your battery but also requiring your vehicle to shut down which is not what you want.
In order to avoid (at least dampen) possible impacts, you can increase the Battery enclosure stiffness by using a thicker Battery housing but of course this would penalise your Battery weigh.
AUDI Dakar RS Q e-tron Bottom Battery casing coverage. Image credit: AUDI AG
The solution used by AUDI was to shape around the Battery pack several skid panels made of CFRP structures reinforced by Zylon. The Battery bottom structure is very important because high jumps and the “natural roads” with many stones and rocks will be challenging for the battery casing bottom. The bottom structure consists in an aluminium plate whose role is to absorb as much as possible the impacts and resists to the abrasion. To dampen the shock, an energy-absorbing foam redistributes the stress to the above sandwich structure. AUDI said that “this underbody with its triple protection against impacts and intrusions is 54 mm thick”. Of course, this extra protection can be monitored and changed during evening service at the Bivouac.
In addition, the Battery is constantly monitored by the BMS. Assuming a Battery failure caused by an impact or anything else, the Battery fuses would immediately shut down the Battery. It also can be manually disconnected by the occupants or safety team thanks to MSD buttons (one being inside the cabin).
AUDI Dakar RS Q e-tron Battery protection. Image credit: AUDI AG
In addition, many panels encapsulate the Battery Pack. Nevertheless, it is interesting to notice that these panels are not sealed not even jointly tight together as can be noticed when you look at the side of the car. Presumably, this allows relative motion between protective panels on big jumps and other shocks, preventing cracks linked to overconstraining the elements.
Visualisation of the Battery Pack in real life. The circle shows how carbon panels are not attached together. The dashed line joins together the cooling access for reference. Image credit: AUDI AG (left), Dakar (right)
Battery Cooling
From the official AUDI pictures and photos, it is easy to see that the Battery uses “immersion cooling” (OROVEL Card EV11). From the photo with Carlos Sainz, it is even possible to estimate the size of the cooling pipe thanks to Carlos’s watch.
AUDI revealed that the coolant used is the 3M™ Novec™ Engineered Fluids, a dielectric fluid already used in some Battery cooling application. 3M Novec specs says that “3M Novec Engineered Fluids […] consist of […] fluoroketones and hydrofluoroethers”.
This is, of course, not the first time that Battery immersion cooling is used (see for example the Koenigsegg Regera Battery Pack made by RIMAC). The Novec dielectric fluid has also already been used in immersion cooling systems, notably XING Mobility publicly shared its use in their system.
The Novec coolant was also used in the Formula E Batteries Gen 2, the mandated battery in the AUDI e-tron FE07 Formula E. Novec is very easy to pump and in Formula E, they used a Bosch electric pump.
If immersion cooling sounds “cool” (lol) it also brings a lot of challenges since its capacity to carry heat away is about half that of water, while it is heavier. It is main advantage is that it is not conductive.
Battery characteristics and architecture
Very little information has been shared about the architecture of the AUDI RS Q e-tron. Publicly, we know that cylindrical cells are used but we do not know which one. If I have to make a guess, I would propose the Sony Murata 18650 because they are (to my knowledge) a very good candidate between energy and high C-rate (which happens occasionally, for example in a difficult dune passage with high driving resistance requiring maximum energy over a short time).
AUDI’s Battery has 52 kWh of usable energy. If we assume that the pack is using Murata 18650, this would mean about 5,000 cells. This assumption would lead to a total cell weight of about 230 kg. This is within a realistic range as AUDI indicated that the Battery pack was weighing about 370-375 kg (including the cooling).
No information about the voltage has been given (to my knowledge) but it is for sure at least 800V (likely 880V).
As for the Battery arrangement, AUDI did not reveal anything about the number of modules. You can have a clue if you look carefully at the computer screen in one of AUDI video (at 0:40). I made a screenshot for you, so that you can better see and make your own conclusions.
AUDI Dakar RS Q e-tron Battery architecture. Images credit: AUDI AG
AUDI Battery conception partner
We end up with my last question….AUDI said that they developed this Battery in-house with a Partner but they have revealed nothing about it… What is below is my own assumption and is based on what I have observed in this article.
The AUDI Dakar Battery is very similar to the Formula E Gen 2. I say that because:
Both Batteries are 52 kWh
Both Batteries use immersion cooling with Novec fluid
AUDI is using their Formula E electric motor(s) MGU05 (including the embedded the inverter). I assume that given the very short development time, they reused as much existing material as possible.
Nevertheless, you will argue that the packaging is very different. I agree but modules and architecture (parallel vs series, etc.) can be adapted without changing the cells (the immersion cooling simplifying this adaptation).
Now, I let you guess who was in charge of the Formula E Gen 2 Battery: McLaren (and yes I know behind McLaren was Atieva, belonging to Lucid)… Nevertheless, if we assume that the Formula E Battery basis was used and only repackaged to fit the AUDI RS Q e-tron, McLaren knowledge of the Battery remains a key advantage.
Not convinced by this theory? I just would like to remind you that AUDI was in discussion at the end of 2021 to partner with McLaren (at some point rumours even suggested that AUDI might buy McLaren) for racing purpose. Coincidence? Maybe…maybe not…
